Add new rule ES.3 Don't repeat yourself (#1715)

* Add new rule ES.3 Don't repeat yourself

* Replace function example with more inclusive/neutral language

Add a reference to topic on libraries.

CppCoreGuidelines.md

@@ -9832,6 +9832,7 @@ General rules:
 
 * [ES.1: Prefer the standard library to other libraries and to "handcrafted code"](#Res-lib)
 * [ES.2: Prefer suitable abstractions to direct use of language features](#Res-abstr)
+* [ES.3: Don't repeat yourself, avoid redundant code](#Res-DRY)
 
 Declaration rules:
 
@@ -9976,6 +9977,44 @@ Once the checking for overflow and error handling has been added that code gets
 
 Not easy. ??? Look for messy loops, nested loops, long functions, absence of function calls, lack of use of non-built-in types. Cyclomatic complexity?
 
+### <a name="Res-DRY"></a>ES.3: Don't repeat yourself, avoid redundant code
+Duplicated or otherwise redundant code obscures intent and makes it harder to understand the logic. There are several other reasons to avoid redundant code. For example, it makes maintenance harder.
+
+Use standard algorithms where apropriate, instead of writing some own implementation.
+
+**See also**: [SL.1](#Rsl-lib)
+
+
+##### Example
+
+    void func(bool flag)    // Bad, duplicated code.
+    {
+        if (flag) {
+            x();
+            y();
+        }
+        else {
+            x();
+            z();
+        }
+    }
+
+    void func(bool flag)    // Better, no duplicated code.
+    {
+        x();
+
+        if (flag)
+            y();
+        else
+            z();
+    }
+
+
+##### Enforcement
+
+* Use a static analyzer. It will catch at least some redundant constructs.
+* Code review
+
 ## ES.dcl: Declarations
 
 A declaration is a statement. A declaration introduces a name into a scope and might cause the construction of a named object.